Volumetric displays belong to a class of three-dimensional display technologies that produce volume-filling imagery. The volume is usually divided into identical volume elements or voxels placed on a grid. This is analogous to pixels in a two dimensional display. The 3D image is constructed by activating any number of voxels. The display is usually controlled by a computer that can make moving images in colour. Volumetric displays are autostereoscopic so viewers do not need any specialised equipment like eyewear or headsets to view a three dimensional image. Furthermore, the image should be visible by any number of viewers.
A common category of volumetric displays includes closed volume displays, which use the principles of swept-volume projection. These are based on mechanical methods such as a periodically moving screen, either rotating or back and forth, which is illuminated by a two dimensional projector that is synchronised with the motion of the screen. As the screen sweeps through the volume, many points in the volume can be illuminated, thereby presenting the human eye with the illusion of a volumetric display. Some of these displays depend on high-speed mechanical components such as a spinning semi-reflective screen whereas other non-mechanical approaches rely on switched stacked liquid crystal screens. Other closed volume technologies range from stacks of organic light emitting diodes or stacked electroluminescent screens to volumes of gas or plasma where the intersections of laser beams create voxels.
Open volume display technologies generate voxels in free space. An example of such a prior art display is based on a flat panel with a microlens array that creates voxels by focusing images at different locations in front of the screen. This technology relies on active matrix semiconductor fabrication techniques.
Prior art indicates that best three-dimensional images are produced by holograms. A hologram is a photosensitive film or plate, which stores diffraction patterns that cause incident light to interfere constructively and destructively to create a three dimensional image outside the plane of the hologram. The hologram stores frequency, amplitude and phase information such that it can reconstruct a three dimensional image when it is suitably illuminated by a light source. The main drawback with holograms is that they can only produce static 3D images. Limited animation is possible with sequences of fixed static 3D images multiplexed on the hologram where the illusion of motion is given by the relative movement of an observer to the hologram. Current holographic printing technologies are capable of producing full colour full parallax 3D images, which are viewable under normal indoor lighting conditions.
A three-dimensional volumetric display is presented in PCT/US00/34466 by Zhan He of Revo Inc, USA. This display is based on a flat panel with a microlens array that creates voxels by focusing images at different locations in front of the screen. The microlens array may be fabricated on a flat panel using active matrix semiconductor fabrication techniques. The present invention does not rely on such focusing or fabrication techniques.
Conventional approaches to holographic displays have focused on high-resolution 3D imaging based on spatial light modulation (SLM) technology. SLMs are digitally programmable diffractive holographic elements that can modulate the phase and/or amplitude of the coherent light passing through it. The diffraction pattern corresponding to the 3D image is first computed and then transferred to the SLM. These are also known as Computer-Generated Holographic (CGH) displays. Examples of SLM-based approaches to holographic displays are described in EP 0992163 Autostereoscopic Display by A. Travis of Cambridge University, UK and IEEE Computer Magazine 2005, article ‘Computer Generated Holography as a Generic Display Technology’, by Chris Slinger et al, Qinetiq Malvern, UK. SLMs are active components that need to be either optically or electronically addressed. Ideally, the SLM needs to behave like a programmable holographic film. Hence the pixel feature size of the SLM needs to be similar to that of the grains within the holographic film emulsion, thereby requiring nanoscale semiconductor technologies that are several generations away from current micron scale feature sizes found in microdisplays. In addition, the SLM-based approach has substantial drawbacks in terms of the substantial computational power and bandwidth needed to calculate the diffraction patterns and to distribute them. SLM components need to be large area devices but this is not feasible or economic using current semiconductor fabrication techniques. This therefore necessitates the use of arrays of smaller SLMs in order to cover a larger area, thus further compounding the bandwidth issue as well as introducing further mechanical and optical problems in mounting the array. Hence, CGH displays based on SLMs or SLM arrays are not commercially viable for cost, performance and technology reasons. Holographic displays based on SLMs are high resolution. The present invention is fundamentally different from SLM approaches in that it is a low-resolution display and it does not depend on SLMs or any active component. Instead, it uses a passive holographic screen, e.g. a film, plate, silica substrate or photopolymer, to record a set of pre-defined 3D images or volume elements, which can be selected individually.
A segmented 3-D hologram display is presented in PCT/US87/03022 by G. Moss of Hughes Aircraft Company, USA. The display consists of several individual holograms embedded in an automobile windshield with each hologram capable of producing segments of a seven segment display. It enables the driver to view digits which are reconstructed outside the plane of the windshield. The holograms are either edge-lit or selectively illuminated. The present invention is a general purpose volumetric holographic display that has applications in many commercial and industrial fields. The invention is based on generic volume elements or voxels, rather than 2D style display segments. The invention uses a holographic screen where the sub-holograms are spread across the surface of the holographic screen, rather than a collection of single holograms.